Using electrogram recordings of the A-V conducting system in man and dogs, in conjunction with atrial and ventricular pacing techniques, it is planned to study the following: 1. Reentry within the His-Purkinje System (man and dog): A. Effect of drugs and cycle length changes; B. Relationship of bundle branch block to HPS reentry; C. Pathway of reentry (in dogs). 2. Relationship of drugs and cycle length changes to retrograde gap phenomena. 3. Role of manifest and concealed reentry in antegrade A-V nodal Wenckebach cycles and 2:1 A-V nodal block. 4. Longitudinal studies of the functional reserve of the HPS in asymptomatic patients with RBBB-LAD.